Fast Oxide-ion Conductor Research Articles

On the Flexibility of the Structural Framework of Cubic LAMOX Compounds, in Relationship with Their Anionic Conduction Properties

A mathematical analysis of the cubic crystal structure of fast oxide-ion conductor beta-La(2)Mo(2)O(9) (and derived members of the LAMOX family) shows that its cationic sublattice can behave as a semirigid framework. Tilt/rotation of rigid [O1La(3)Mo] anti-tetrahedral units about their 3-fold axis can open up tunnels in the cationic framework, therefore favoring the mobility of O2 and O3 oxide ions located in these tunnels, as confirmed by molecular dynamics simulations. Such a process is likely to assist the anionic transport and explain the postulated transition from Arrhenius-type to VTF (Vogel-Tamman-Fulcher)-type behavior propounded to account for the peculiar conductivity curvature observed at high temperature in all the cubic LAMOX compounds. It also clarifies the correlated extra cell volume expansion observed at the same temperature in all these materials.

Visualization of the diffusion path in the fast oxide-ion conductor Bi1.4Yb0.6O3

Accurate nuclear-density distribution of bismuth oxide solution Bi1.4Yb0.6O3 compound has been studied at 384 °C and at 738 °C by the maximum-entropy method (MEM) and MEM-based pattern fitting combined with the Rietveld method using neutron powder diffraction data. The results reveal that the oxide ions have a complicated disorder spreading over a wide area, shift to the ⟨111⟩ directions from the ideal fluorite position and diffuse along the ⟨100⟩ directions.

In situ Neutron Diffraction Study on Fast Oxide Ion Conductor LaGaO3‐Based Perovskite Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effects of Partial Substitution of Mo6+ by Cr6+ and W6+ on the Crystal Structure of the Fast Oxide-Ion Conductor Structural Effects of W6+.

Effects of Partial Substitution of Mo⁶⁺ by Cr⁶⁺ and W⁶⁺ on the Crystal Structure of the Fast Oxide-Ion Conductor Structural Effects of W⁶⁺.

Ionic conduction path and disorder in LaGaO3-based fast oxide-ion conductors

Ionic conduction path and disorder in LaGaO₃-based fast oxide-ion conductors

Structural analysis of La2Mo2O9-based fast oxide-ion conductors

Structural analysis of La₂Mo₂O₉-based fast oxide-ion conductors

Effects of Partial Substitution of Mo6+ by Cr6+ and W6+ on the Crystal Structure of the Fast Oxide-Ion Conductor Structural Effects of W6+

The crystal structure of the solid solutions La2Mo2-yCryO9 (y ≤ 0.5) and La2Mo2-yWyO9 (y ≤ 1.4) has been studied using X-ray and neutron powder diffraction. These two series of lanthanum molybdates, which belong to the LAMOX family of fast oxide-ion conductors, exhibit a different structural behavior depending on the substituting element. The Cr series follows a regular Vegard-type evolution of crystallographic parameters. However, the behavior of the W series is different, the lattice constant varying in a nonlinear fashion with substitution level, resulting in a smaller cell volume for higher tungsten contents, despite the larger ionic radius of tungsten. Two main structural effects are evidenced: a variation in the distribution of ligands around tungsten, made apparent through the changing balance of oxygen O2 and O3 site occupancies along the series, and a nonlinear evolution of interatomic distances and angles involving the O1 oxygen site. An alternative structural description, based on [O1La3Mo] an...

Physicochemical compatibility of CGO fluorite, LSM and LSCF perovskite electrode materials with La 2Mo 2O 9 fast oxide-ion conductor

The reactivity of typical electrode materials for solid oxide fuel cells, namely La 0.6Sr 0.4Co 0.2Fe 0.8O 3− δ (LSCF), La 0.8Sr 0.2MnO 3− δ (LSM) and Ce 0.9Gd 0.1O 1.95 (CGO), with fast oxide-ion conductor La 2Mo 2O 9 is thoroughly studied using X-ray diffractometry. Modifications in the materials diffraction patterns and occurrence of impurity diffraction peaks are evidenced in LSCF and LSM above 700–800 °C. They are interpreted in terms of cationic migrations from one material phase to the other (typically strontium depletion/substitution), and subsequent formation of subsidiary phases (for instance strontium molybdate). With CGO no reactivity is observed up to 1000 °C. For all three electrode materials the thermal expansion coefficient (TEC) is measured from X-ray thermodiffractograms and compared to that of La 2Mo 2O 9.

Synthesis by sol–gel route of oxyapatite powders for dense ceramics: Applications as electrolytes for solid oxide fuel cells

Solid oxide fuel cells have considerable interest in recent years, because of their high efficiency and environmentally friendly nature. Such systems required oxygen-conducting electrolytes and now the most common electrolyte is yttria stabilized zirconia (YSZ). This compound exhibits high oxide ion conductivity at elevated temperatures (850–1000 °C). However, this high working temperature causes problems in terms of materials selection and lifetime. One solution is to develop new oxide ions conductors exhibiting high oxide ion conductivity at intermediary temperatures (700–800 °C). Recent work has identified Ln 10− x Si 6O 26± z (Ln = rare earths) as a good fast oxide ion conductor. Undoped and doped Ln 10− x B 6O 26± z (B = Si or Ge) oxides are currently prepared by solid-state methods. In that work, we propose a sol–gel process to synthesize powders of La 9.33Si 6O 26 type-silicated apatites. The main advantage is to decrease the crystallization temperature in comparison to the conventional methods, allowing the synthesis of reactive powders with nanometric particles size. These oxides are synthesized using silicon alkoxide and lanthanum nitride as precursors. In the litterature, no study refers to the synthesis of mixed oxides with silicon alcoxides. However, there are several studies on sol–gel synthesis of glasses with this precursor. In this study, several processing parameters have been investigated (the hydrolysis ratio, the concentration of metallic precursors in the sol and the role of organic compounds) in order to synthesize pure phases after the decomposition of the sols. Pure powders of La 9.33Si 6O 26 type-silicated apatites are obtained at 800 °C. These powders were used to prepare ceramics. Several processing parameters as morphology of powders (agglomeration, particle sizes) and, heating profiles have been studied on the densification. Dense ceramics (90–95%) have been prepared at temperatures around 1400 °C. The used of sol–gel powders allow the decrease of the sintering temperature of about 200 °C.

Sintering and electrical conductivity in fast oxide ion conductors La 2− xR xMo 2− yW yO 9 (R: Nd, Gd, Y)

Attrition and ball milling are used as mechanical means to reduce grain size of optimized fast oxide-ion conductors La 2− x R x Mo 2− y W y O 9 (R: rare earths). Dilatometry is used to determine the optimal sintering conditions in order to obtain high density samples (greater than 96% of relative density) with help of scanning electron microscopy to characterize their microstructure. The optimal sintering temperatures are highly dependent on the chemical composition, and therefore identical annealing temperatures do not warrant similar relative densities. Complex impedance spectroscopy show that above the transition temperature of La 2Mo 2O 9 at 580 °C, the conductivity of all the studied compounds is lower than that of the parent compound, whereas just below the transition, in most cases the stabilization of the cubic phase increases conductivity. An interesting result is that tungsten substitution, which stabilizes La 2Mo 2O 9 against reduction, does not affect significantly the oxide ion conduction.

The LAMOX Family of Fast Oxide‐Ion Conductors: Overview and Recent Results

AbstractFor Abstract see ChemInform Abstract in Full Text.

Conduction path and disorder in the fast oxide-ion conductor (La 0.8Sr 0.2)(Ga 0.8Mg 0.15Co 0.05)O 2.8

We analyzed the nuclear density distribution in the fast oxide-ion conductor (La 0.8Sr 0.2)(Ga 0.8Mg 0.15Co 0.05)O 2.8 to explore the conduction path and disorder of oxide ions at high temperatures. The conduction path of oxide ions was not along the straight line between the ideal positions, but exhibited an arc shape away from the site of Ga 0.8Mg 0.15Co 0.05 cations. In the low-temperature rhombohedral phase, the oxide ions were localized near the equilibrium positions, while in the high-temperature cubic structure, they spread over a wide area between the ideal positions.

Novel fast oxide ion conductor and application for the electrolyte of solid oxide fuel cell

Effects of Co doping to Ga sites of a LaGaO 3 based oxide on the oxide ion conductivity was investigated. Oxide ion conductivity increased by doping with Co and it was found that usage of a LaGaO 3-based perovskite type oxide, doped with Sr for the A site and Co and Mg for the B site (La 0.8Sr 0.2Ga 0.8Mg 0.115Co 0.085O 3 denoted as LSGMC), for the electrolyte of the fuel cell gave a notably large power density at an intermediate temperature of 873 K on a cell using H 2 and O 2 as fuel and oxidant, respectively. The power density increased as the thickness of the electrolyte was decreased. The maximum power density was attained at values of 1.4 and 0.5 W/cm 2 at 1073 and 873 K, respectively, when 0.18 mm thick LSGMC was used for the electrolyte. Electrical conductivity in the LSGMC was also estimated using polarization methods. Electrical conductivity was also increased by doping with Co, resulting in an increased amount of chemically leaked oxygen. Consequently, the theoretical calculation demonstrated that the highest energy conversion efficiency would be achieved when the thickness of the LSGMC electrolyte was 100 μm.

Crystal structure and disorder of the fast oxide-ion conductor cubic Bi 2O 3

Accurate nuclear density distribution of cubic bismuth oxide (δ-Bi 2O 3) has been obtained by the maximum entropy method (MEM)-based pattern fitting combined with the Rietveld method using neutron-powder-diffraction data. The result shows for the first time that the oxide ions have a complicated disorder spreading over a wide area and shift to the 〈1 1 1〉 directions from the ideal fluorite site. These features are responsible for the high oxide-ion conductivity.

Thermal, structural and transport properties of the fast oxide-ion conductors La 2−xR xMo 2O 9 (R=Nd, Gd, Y)

The thermal evolution of the crystal structure of the new fast oxide-ion conductor La 2Mo 2O 9 across its structural α to β first order transition at 580 °C has been followed using synchrotron X-ray diffraction (XRD). The influence on the transition, structure and oxide-ion conductivity of partial substitution by other rare earths (Nd, Gd, Y) is studied. For Nd substitution the monoclinic α form is retained at room temperature in the whole compositional range. For Gd and Y substitutions, above a certain doping level, the suppression of the phase transition and stabilization at room temperature of the cubic β form are achieved. The cubic structures of La 1.9Y 0.1Mo 2O 9 at room temperature and of β-La 2Mo 2O 9 at 670 °C (revisited), both refined from neutron powder diffraction patterns, are very similar. Despite the cell volume reduction, lanthanide substitution results above 500 °C in an increase of the anion conductivity compared to that of β-La 2Mo 2O 9, which might result from differences in samples' relative density.

Ionic conductivity of the oxide family Bi[Bi 12O 14][(Mo,M)O 4] 5 with M=Li, Mg, Al, Si, Ge and V

The structure and electric conductivity of the fast oxide ion conductors Bi 13(Mo,M) 5O 34, i.e. Bi[Bi 12O 14][(Mo,M)O 4] 5 with M=Li, Mg, Al, Si, Ge and V were investigated using X-ray powder diffraction experiments and AC impedance spectroscopy. The compounds of this original family are isostructural with Bi 13Mo 4VO 34; they crystallize in the monoclinic system with the space group P2/ c. The structures exhibit typical [Bi 12O 14] n columns developed along [010], surrounded by (Mo,M)O 4 tetrahedra packed in layers parallel to (100) and (001), the electrical balance of the network being achieved by Bi atoms at the intersection of these planes. The syntheses, X-ray powder pattern analyses and conductivity measurements are reported; the best anionic conductor of the family is proved to be Bi[Bi 12O 14][(Mo 0.933Al 0.067)O 4] 5 with σ 980 K=1.4×10 −2 S cm −1.

Reducibility of fast oxide-ion conductors La2−xRxMo2−yWyO9(R = Nd, Gd)

The substitutional range and cell parameter evolution of fast oxide-ion conductors La2−xRxMo2−yWyO9 (R = Nd, Gd) are investigated. In the whole series, the cubic β-La2Mo2O9 structural type is stabilized at room temperature. The effects on reducibility of both single and double substitutions are presented. Lanthanum substitution by rare earth appeared to be responsible for an increase in the reducibility and a strong but reversible amorphization under dilute hydrogen. On the contrary, the favourable role of tungsten on the compound stability under reducing conditions is evidenced: it depletes oxygen loss while making the La2Mo2O9 structural type less affected by it.

Fast oxide ion conductivity and oxygen tracer diffusion in doped La 2GeO 5− δ

Oxide ion conductivity in La-deficient La 2GeO 5−δ was investigated with 18O tracer diffusion measurement. It was found that La 1.61GeO 4.415 exhibits a large diffusion coefficient, which is in good agreement with that estimated from electrical conductivity. Therefore, it was confirmed that La-deficient La 2GeO 5 is a pure oxide ionic conductor. The effects of doping alkaline earth cations to the La site of La 1.8GeO 4.7 were also investigated. Although the crystal structure was slightly changed, alkaline earth cations seem to substitute for the La site in La 2GeO 5. The electrical conductivity at low temperature is increased by doping alkaline earth cations, in particular, it was found that doping with Ca is the most effective for increasing the electrical conductivity. The electrical conductivity increased with increasing Ca and attained the maximum at a composition of La 1.5Ca 0.2GeO 4.45. The transport number of the oxide ion, estimated by H 2–O 2 cell, was almost 1.0. Therefore, Ca-doped La 2GeO 5 oxide is also a pure oxide ionic conductor. Since the electrical conductivity of La 1.5Ca 0.2GeO 4.45 was higher than that of Zr 0.84Y 0.15O 2 at all temperature examined, Ca-doped La 2GeO 5 based oxide is attractive as a new fast oxide ion conductor.

Structural and electrochemical features of fast oxide ion conductors

New fast oxide ion conductors for low temperature applications (400–600°C) have been proposed during recent years. These materials exhibit structural features rather different from that of the well-known zirconia-based electrolytes. The aim of this paper is to analyse the main structural characteristics of these various classes of materials to propose them as a guideline for designing new oxide ion conductors. The development of efficient low temperature devices also implies association of a fast electrolyte to a very active electrode system which must be able to support oxygen molecule dissociation and recombination at high current densities. The BIMEVOX materials offer the unique opportunity of being able to act both as the electrolyte and the electrodes, overcoming many chemical and mechanical problems. This point will also be discussed.

ChemInform Abstract: Nonstoichiometric La2‐xGeO5‐x Monoclinic Oxide as a New Fast Oxide Ion Conductor.

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